- Email: [email protected]
IrMn trilayer structure
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e718–e720 www.elsevier.com/locate/jmmm
Influence of spacer layer in exchange coupled NiFe/Cu/IrMn trilayer structure Yong-Goo Yooa,, Seong-Gi Minb, Seong-Cho Yub a
Electronics and Telecommunications Research Institute, Daejeon 305-350, Korea Department of Physics, Chungbuk National University, Cheongju 361-763, Korea
b
Available online 20 March 2006
Abstract Variations in exchange coupling for NiFe/Cu/IrMn trilayer thin films were examined as a function of the Cu spacer thickness. The exchange coupling field and an uniaxial anisotropy field were analyzed by ferromagnetic resonance measurements with an angular variation. Angular dependence of resonance field showed unidirectional behavior but the film with thick Cu layer showed a similar trend to uncoupled NiFe single layer film. As Cu spacer thickness increase, the exchange coupling field and the uniaxial anisotropy field decreased with different decrease trend from Cu spacer thickness of 0.8 nm. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 68:55: a Keywords: Exchange coupling; FMR; Uniaxial anisotropy
1. Introduction
2. Experimentals
Considerable research has been recently devoted to the study of exchange coupling phenomena, from the point of view of their potential applications to spin valve structures [1]. One of research on exchange coupling, a coupling range between ferromagnetic (FM) and antiferromagnetic (AFM) layers gives interesting results in terms of understanding the exchange coupling effect [2,3]. A coupling range study of FM/AFM structures can be carried out by using the insertion of a spacer between the FM and AFM layers [3]. Ferromagnetic resonance (FMR) measurement represents a powerful tool for studying the magnetic anisotropy which can be determined by the intrinsic property of the FM layer and/or an exchange coupling in the case of an exchange biased thin film [4]. In this study, we report on the influence of spacer layer in exchange coupled NiFe/Cu/IrMn thin films with a Cu spacer inserted at the interface between the FM and AFM layer. Unidirectional exchange anisotropy and the uniaxial anisotropy were analyzed by FMR measurements.
The exchange coupled NiFe (5 nm)/Cu(t)/ IrMn (10 nm) thin films were fabricated by magnetron sputtering on a thermally oxidized Si(1 0 0) wafer with a seed layer of Ta (5 nm). During the deposition, a magnetic field of 200 Oe was applied in order to form an uniaxial anisotropy in the FM layer. The thickness of the Cu spacer was varied at 0.2, 0.4, 0.8, 1.2, 1.4, and 1.8 nm. A single layer sample of NiFe (5 nm) was also fabricated for comparison. Ta (5 nm) was deposited on the top of all samples, to prevent oxidation. The base pressure of the chamber was below 5 108 torr and an Ar working pressure was 2 mtorr. The FMR measurements were performed at 9.4 GHz (X band) with a JEOL JES-TE300 ESR Spectrometer. The angular dependence of the resonance field ðH r Þ was measured by rotating the sample to an in plane direction with respect to the applied magnetic field.
Corresponding author. Tel.: +82 42 860 5222; fax: +82 42 860 5202.
The angular dependence of the resonance field for the exchange coupled thin films was determined by FMR
E-mail address: [email protected] (Y.-G. Yoo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.201
3. Results and discussion
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e718–e720
H r ðjH Þ ¼ H r0 H ex cos jH H k cos 2jH ,
1020
Hr (Oe)
1000
980 NiFe
960
940 0
60
0.2 nm
1.2 nm
0.4 nm
1.4 nm
0.8 nm
1.8 nm
120
180
240
300
360
φH(H) Fig. 1. Angular dependence of the H r for an exchange biased NiFe (5 nm)/Cu(t)/IrMn (10 nm) thin films. NiFe denotes NiFe (5 nm) single layer thin film. The solid lines indicate data fitted to Eq. (1).
Hex Hk
5.5
Hk(NiFe)
5.0
30
4.5 20
Hk (Oe)
40
4.0 10 3.5 0 0.0
(1)
0.4
0.8
1.2
3.0 2.0
1.6
Cu t (nm) Fig. 2. Variation of unidirectional anisotropy field and uniaxial anisotropy field as a function of Cu spacer thickness. The solid lines indicate eye guide.
65
60
∆H (Oe)
where jH indicates the angle between the applied field direction and the easy axis of FM layer. H r0 is an average resonance field which can be expressed by H r0 ¼ ðo=gÞ2 ð1=4pM F Þ, where o, g, and M F are an angular frequency, a gyromagnetic ratio and FM magnetization, respectively. The H r0 indicates angular-independent term. The second term of Eq. (1) represents the exchange coupling between FM and AFM layer and the third term describes the uniaxial anisotropy of the FM layer. The angular variation of H r for NiFe/Cu/IrMn thin films was well fitted by Eq. (1) as can be seen by a solid line in Fig. 1. The variation of H ex and H k , which can be extracted from the angular variation of H r , were shown in Fig. 2. The H ex decreased steeply with the increase of Cu spacer thickness up to Cu 0.8 nm and then it showed a different decrease trend above Cu thickness of 0.8 nm. It should be noted that H ex value is still remained to Cu 1.8 nm inserted thin film corresponding to the presence of the long-range ordering of exchange coupling in the Cu spacer inserted FM/AFM structure. In case of the films with thin Cu spacer, the exchange coupling could took place by possibility of direct interfacial contact through pinholes while the films with thick Cu spacer were exchange coupled across continuous Cu spacer layer [6]. This different interfacial structure gives rise to the different decrease
6.0
50
Hex (Oe)
measurements. Fig. 1 shows the angular variation in H r with Cu spacer thickness. The angular variation in H r indicates a manifest unidirectional behavior for the thin Cu spacer inserted films. On the other hand, the H r curve for the thick Cu spacer inserted films exhibit similar behavior to the NiFe single layer thin film, corresponding to weak and long-range exchange coupling through Cu spacer layer. An unidirectional anisotropy field ðH ex Þ and an uniaxial anisotropy field ðH k Þ can be extracted by the phenomenological expression [5] of cosine series neglecting high-order term as follows:
e719
55
50
45 NIFe 40 0.0
0.4
0.8
1.2
1.6
2.0
Cu t (nm) Fig. 3. The mean peak-to-peak line width variation of exchange coupled NiFe (5 nm)/Cu(t)/IrMn (10 nm) thin films as a function of Cu spacer thickness. NiFe indicates NiFe (5nm) single layer thin film. The solid line indicates eye guide.
trend of H ex with Cu spacer thickness. The variation of H k shows similar trend to the H ex variation. It should be noted that the H k value for the film with thin Cu spacer is larger than that of the unbiased NiFe single layer thin film while the films with thick Cu spacer are comparable to NiFe film, as shown in Fig. 2. It means that the uniaxial anisotropy of exchange coupled NiFe film could be associated with the exchange coupling [7,8]. In addition to the resonance field distribution in FMR measurement, a line broadening gives useful information to study of exchange coupled structure. The line broadening in FMR measurement comes from intrinsic properties such as anisotropy, structural defects, measuring frequency and inhomogeneous coupling in case of multilayer [1]. Fig. 3 shows the mean peak-to-peak line width variation of exchange coupled NiFe (5 nm)/Cu(t)/IrMn (10 nm) thin films as a function of Cu spacer thickness. With increasing
ARTICLE IN PRESS e720
Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e718–e720
Cu thickness, the line width linearly decreased up to Cu 0.8 nm and then it showed a somewhat different decrease trend in films with thick Cu spacer, which agrees with H ex and H k variation. Commonly, the exchange biased thin film exhibits larger line broadening than that of unbiased single layer thin film [9]. It was reported that the line broadening of exchange biased film is caused by a local pinning of FM layer due to an inhomogeneous surface anisotropy based on Malozemoff’s model [10]. Thus, the amount of line broadening varies in proportion to exchange coupling strength. 4. Conclusion The Cu spacer layer inserted NiFe/IrMn thin films were studied by FMR measurement. The exchange coupling field and the uniaxial anisotropy were analyzed with the angular variation of resonance field. The thin Cu spacer inserted thin films showed unidirectional behavior while thick Cu spacer inserted thin films exhibit the similar behavior to NiFe single layer thin film. With Cu thickness increase, the variation of H ex and H k showed different
decrease trend around Cu 0.8 nm. The variation of line width showed similar behavior to H ex and H k variation with Cu thickness. References [1] M. Kiwi, J. Magn. Magn. Mater. 234 (2001) 584. [2] N.J. Gokemeijer, T. Ambrose, C.L. Chien, Phys. Rev. Lett. 79 (1997) 4270. [3] L. Thomas, A.J. Kellock, S.S.P.P. Parkin, J. Appl. Phys. 87 (2000) 5061. [4] J. Lindner, K. Baberschke, J. Phys. Condens. Matter 15 (2003) R193. [5] A. Layadi, W.C. Cain, J.-W. Lee, J.O. Artman, IEEE Trans. Magn. MAG-23 (1987) 2993. [6] S. Mao, Z. Gao, H. Xi, P. Kolbo, M. Plumer, L. Wang, A. Goyal, I. Jin, J. Chen, C. Hou, R.M. White, IEEE Trans. Magn. 38 (2002) 26. [7] H. Xi, R.M. White, S.M. Rezende, Phys. Rev. B 60 (1999) 14837. [8] R. Shan, W.W. Lin, L.F. Yin, C.S. Tian, H. Sang, L. Sun, S.M. Zhou, Phys. Rev. B 71 (2005) 64402. [9] R.D. McMichael, M.D. Stiles, P.J. Chen, W.F. Egelhoff Jr., J. Appl. Phys. 83 (11) (1998) 7037. [10] V.S. Speriosu, S.S.P. Parkin, IEEE Trans. Magn. MAG-23 (1987) 2999.
Journal of Magnetism and Magnetic Materials 304 (2006) e718–e720 www.elsevier.com/locate/jmmm
Influence of spacer layer in exchange coupled NiFe/Cu/IrMn trilayer structure Yong-Goo Yooa,, Seong-Gi Minb, Seong-Cho Yub a
Electronics and Telecommunications Research Institute, Daejeon 305-350, Korea Department of Physics, Chungbuk National University, Cheongju 361-763, Korea
b
Available online 20 March 2006
Abstract Variations in exchange coupling for NiFe/Cu/IrMn trilayer thin films were examined as a function of the Cu spacer thickness. The exchange coupling field and an uniaxial anisotropy field were analyzed by ferromagnetic resonance measurements with an angular variation. Angular dependence of resonance field showed unidirectional behavior but the film with thick Cu layer showed a similar trend to uncoupled NiFe single layer film. As Cu spacer thickness increase, the exchange coupling field and the uniaxial anisotropy field decreased with different decrease trend from Cu spacer thickness of 0.8 nm. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 68:55: a Keywords: Exchange coupling; FMR; Uniaxial anisotropy
1. Introduction
2. Experimentals
Considerable research has been recently devoted to the study of exchange coupling phenomena, from the point of view of their potential applications to spin valve structures [1]. One of research on exchange coupling, a coupling range between ferromagnetic (FM) and antiferromagnetic (AFM) layers gives interesting results in terms of understanding the exchange coupling effect [2,3]. A coupling range study of FM/AFM structures can be carried out by using the insertion of a spacer between the FM and AFM layers [3]. Ferromagnetic resonance (FMR) measurement represents a powerful tool for studying the magnetic anisotropy which can be determined by the intrinsic property of the FM layer and/or an exchange coupling in the case of an exchange biased thin film [4]. In this study, we report on the influence of spacer layer in exchange coupled NiFe/Cu/IrMn thin films with a Cu spacer inserted at the interface between the FM and AFM layer. Unidirectional exchange anisotropy and the uniaxial anisotropy were analyzed by FMR measurements.
The exchange coupled NiFe (5 nm)/Cu(t)/ IrMn (10 nm) thin films were fabricated by magnetron sputtering on a thermally oxidized Si(1 0 0) wafer with a seed layer of Ta (5 nm). During the deposition, a magnetic field of 200 Oe was applied in order to form an uniaxial anisotropy in the FM layer. The thickness of the Cu spacer was varied at 0.2, 0.4, 0.8, 1.2, 1.4, and 1.8 nm. A single layer sample of NiFe (5 nm) was also fabricated for comparison. Ta (5 nm) was deposited on the top of all samples, to prevent oxidation. The base pressure of the chamber was below 5 108 torr and an Ar working pressure was 2 mtorr. The FMR measurements were performed at 9.4 GHz (X band) with a JEOL JES-TE300 ESR Spectrometer. The angular dependence of the resonance field ðH r Þ was measured by rotating the sample to an in plane direction with respect to the applied magnetic field.
Corresponding author. Tel.: +82 42 860 5222; fax: +82 42 860 5202.
The angular dependence of the resonance field for the exchange coupled thin films was determined by FMR
E-mail address: [email protected] (Y.-G. Yoo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.201
3. Results and discussion
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e718–e720
H r ðjH Þ ¼ H r0 H ex cos jH H k cos 2jH ,
1020
Hr (Oe)
1000
980 NiFe
960
940 0
60
0.2 nm
1.2 nm
0.4 nm
1.4 nm
0.8 nm
1.8 nm
120
180
240
300
360
φH(H) Fig. 1. Angular dependence of the H r for an exchange biased NiFe (5 nm)/Cu(t)/IrMn (10 nm) thin films. NiFe denotes NiFe (5 nm) single layer thin film. The solid lines indicate data fitted to Eq. (1).
Hex Hk
5.5
Hk(NiFe)
5.0
30
4.5 20
Hk (Oe)
40
4.0 10 3.5 0 0.0
(1)
0.4
0.8
1.2
3.0 2.0
1.6
Cu t (nm) Fig. 2. Variation of unidirectional anisotropy field and uniaxial anisotropy field as a function of Cu spacer thickness. The solid lines indicate eye guide.
65
60
∆H (Oe)
where jH indicates the angle between the applied field direction and the easy axis of FM layer. H r0 is an average resonance field which can be expressed by H r0 ¼ ðo=gÞ2 ð1=4pM F Þ, where o, g, and M F are an angular frequency, a gyromagnetic ratio and FM magnetization, respectively. The H r0 indicates angular-independent term. The second term of Eq. (1) represents the exchange coupling between FM and AFM layer and the third term describes the uniaxial anisotropy of the FM layer. The angular variation of H r for NiFe/Cu/IrMn thin films was well fitted by Eq. (1) as can be seen by a solid line in Fig. 1. The variation of H ex and H k , which can be extracted from the angular variation of H r , were shown in Fig. 2. The H ex decreased steeply with the increase of Cu spacer thickness up to Cu 0.8 nm and then it showed a different decrease trend above Cu thickness of 0.8 nm. It should be noted that H ex value is still remained to Cu 1.8 nm inserted thin film corresponding to the presence of the long-range ordering of exchange coupling in the Cu spacer inserted FM/AFM structure. In case of the films with thin Cu spacer, the exchange coupling could took place by possibility of direct interfacial contact through pinholes while the films with thick Cu spacer were exchange coupled across continuous Cu spacer layer [6]. This different interfacial structure gives rise to the different decrease
6.0
50
Hex (Oe)
measurements. Fig. 1 shows the angular variation in H r with Cu spacer thickness. The angular variation in H r indicates a manifest unidirectional behavior for the thin Cu spacer inserted films. On the other hand, the H r curve for the thick Cu spacer inserted films exhibit similar behavior to the NiFe single layer thin film, corresponding to weak and long-range exchange coupling through Cu spacer layer. An unidirectional anisotropy field ðH ex Þ and an uniaxial anisotropy field ðH k Þ can be extracted by the phenomenological expression [5] of cosine series neglecting high-order term as follows:
e719
55
50
45 NIFe 40 0.0
0.4
0.8
1.2
1.6
2.0
Cu t (nm) Fig. 3. The mean peak-to-peak line width variation of exchange coupled NiFe (5 nm)/Cu(t)/IrMn (10 nm) thin films as a function of Cu spacer thickness. NiFe indicates NiFe (5nm) single layer thin film. The solid line indicates eye guide.
trend of H ex with Cu spacer thickness. The variation of H k shows similar trend to the H ex variation. It should be noted that the H k value for the film with thin Cu spacer is larger than that of the unbiased NiFe single layer thin film while the films with thick Cu spacer are comparable to NiFe film, as shown in Fig. 2. It means that the uniaxial anisotropy of exchange coupled NiFe film could be associated with the exchange coupling [7,8]. In addition to the resonance field distribution in FMR measurement, a line broadening gives useful information to study of exchange coupled structure. The line broadening in FMR measurement comes from intrinsic properties such as anisotropy, structural defects, measuring frequency and inhomogeneous coupling in case of multilayer [1]. Fig. 3 shows the mean peak-to-peak line width variation of exchange coupled NiFe (5 nm)/Cu(t)/IrMn (10 nm) thin films as a function of Cu spacer thickness. With increasing
ARTICLE IN PRESS e720
Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e718–e720
Cu thickness, the line width linearly decreased up to Cu 0.8 nm and then it showed a somewhat different decrease trend in films with thick Cu spacer, which agrees with H ex and H k variation. Commonly, the exchange biased thin film exhibits larger line broadening than that of unbiased single layer thin film [9]. It was reported that the line broadening of exchange biased film is caused by a local pinning of FM layer due to an inhomogeneous surface anisotropy based on Malozemoff’s model [10]. Thus, the amount of line broadening varies in proportion to exchange coupling strength. 4. Conclusion The Cu spacer layer inserted NiFe/IrMn thin films were studied by FMR measurement. The exchange coupling field and the uniaxial anisotropy were analyzed with the angular variation of resonance field. The thin Cu spacer inserted thin films showed unidirectional behavior while thick Cu spacer inserted thin films exhibit the similar behavior to NiFe single layer thin film. With Cu thickness increase, the variation of H ex and H k showed different
decrease trend around Cu 0.8 nm. The variation of line width showed similar behavior to H ex and H k variation with Cu thickness. References [1] M. Kiwi, J. Magn. Magn. Mater. 234 (2001) 584. [2] N.J. Gokemeijer, T. Ambrose, C.L. Chien, Phys. Rev. Lett. 79 (1997) 4270. [3] L. Thomas, A.J. Kellock, S.S.P.P. Parkin, J. Appl. Phys. 87 (2000) 5061. [4] J. Lindner, K. Baberschke, J. Phys. Condens. Matter 15 (2003) R193. [5] A. Layadi, W.C. Cain, J.-W. Lee, J.O. Artman, IEEE Trans. Magn. MAG-23 (1987) 2993. [6] S. Mao, Z. Gao, H. Xi, P. Kolbo, M. Plumer, L. Wang, A. Goyal, I. Jin, J. Chen, C. Hou, R.M. White, IEEE Trans. Magn. 38 (2002) 26. [7] H. Xi, R.M. White, S.M. Rezende, Phys. Rev. B 60 (1999) 14837. [8] R. Shan, W.W. Lin, L.F. Yin, C.S. Tian, H. Sang, L. Sun, S.M. Zhou, Phys. Rev. B 71 (2005) 64402. [9] R.D. McMichael, M.D. Stiles, P.J. Chen, W.F. Egelhoff Jr., J. Appl. Phys. 83 (11) (1998) 7037. [10] V.S. Speriosu, S.S.P. Parkin, IEEE Trans. Magn. MAG-23 (1987) 2999.